This invention relates generally to a two axis centering system particularly useful for positioning a work in an X, Y coordinate plane in accordance with given precise coordinate points. These given precise coordinate points are provided on a master control member such as a template or master art work in order that automatic machining operations can be carried out accurately.
In our U.S. Pat. No. 3,293,439 issued Dec. 20, 1966 and entitled TWO AXIS PHOTOELECTRIC POSITIONING SYSTEM INCLUDING ALTERNATE AXIS ACTIVATION OF PHOTO CELL, there is described a photoelectric means for effecting positioning of a master template at precise rectangular coordinate points. Essentially, a photocell is provided on one side of the master template and a light source on the other, the master template including a hole through which the light passes. Off-centering of the hole changes the light pattern on the photo cell giving rise to signals which operate servo motors to move the master template for art work in a proper direction to exactly center the hole relative to the light path; that is, to minimize the servo signals. These movements also control a circuit board or other work through which a hole is to be drilled at a desired coordinate position.
While the system briefly described above operates satisfactorily, it is found that in utilizing the holes in a master template, there is often a variation in the size or diameter of the hole with the result that consistent operation cannot always be realized. Further, the master template may be defective, such that light passes through a non-circular opening to strike the cell and such could operate the drilling mechanism, thereby drilling a false hole. Finally, for extremely large holes in the master template, off centering of the hole cannot be as accurately detected as is desirable for precision operations.
In our later U.S. Pat. No. 3,716,716 issued Feb. 13, 1973 and entitled "PHOTOELECTRIC TWO AXIS POSITIONING SYSTEM", we disclose an improved photoelectric two axis positioning system developed by us wherein far greater accuracy in precisely centering a hole relative to fixed rectangular coordinates is realizable, even though the hole itself may vary in diameter.
In accord with this improved invention, a member such as a master art work having a circular hole is positioned between a light source and a photo cell, the path of light being on an axis normal to the member and passing precisely through the given coordinate points. A slot is provided between the photo cell and the hole in the member, the slot extending radially from the given axis beyond the boundary of the hole. With this arrangement, it will be evident that by revolving the slot about the axis of the light path, the photo cell will receive a constant amount of light for any revolved position of the slot only when the exact center of the hole in the member coincides with the axis of revolution. Any off-centering of the hole from the axis will result in a variable amount of light passing to the cell over a complete cycle of rotation, thereby enabling a variable signal to be derived from the cell. This signal may be employed to operate servo motors to move the member or master art work in a direction to minimize the variations in the signal and thereby assure the exact centering of the hole in the member at the given coordinate points.
While our foregoing arrangement provides great improvement in precise two axis centering systems, the use of the revolving slot requires a very careful and expensive machining operation to assure mechanical perfection. Moreover, in the event that the light passing through the hole does not provide for precise uniform illumination over the entire area of the hole, false variable signals can be generated by the revolving slot, even though precise centering has been achieved. Finally, the response time in effecting centering involves the detection of the variable signal and conversion of the same into appropriate servo signals and there is consequently some delay in effecting the centering.